{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,20]],"date-time":"2026-10-20T01:53:14Z","timestamp":1792461194751,"version":"4.5.0"},"reference-count":66,"publisher":"American Physiological Society","issue":"2","content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["American Journal of Physiology-Regulatory, Integrative and Comparative Physiology"],"published-print":{"date-parts":[[2008,2]]},"abstract":"<jats:p>\n                    We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the \u201ccritical power\u201d (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using\n                    <jats:sup>31<\/jats:sup>\n                    P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3\u201318 min) for estimation of the CP (mean \u00b1 SD, 20 \u00b1 2 W). Subsequently, the subjects exercised at work rates 10% below CP (&lt;CP) for 20 min and 10% above CP (&gt;CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P\n                    <jats:sub>i<\/jats:sub>\n                    concentration ([P\n                    <jats:sub>i<\/jats:sub>\n                    ]), and pH, were estimated using\n                    <jats:sup>31<\/jats:sup>\n                    P-MRS. All subjects completed 20 min of &lt;CP exercise without duress, whereas the limit of tolerance during &gt;CP exercise was 14.7 \u00b1 7.1 min. During &lt;CP exercise, stable values for [PCr], [P\n                    <jats:sub>i<\/jats:sub>\n                    ], and pH were attained within 3 min after the onset of exercise, and there were no further significant changes in these variables (end-exercise values = 68 \u00b1 11% of baseline [PCr], 314 \u00b1 216% of baseline [P\n                    <jats:sub>i<\/jats:sub>\n                    ], and pH 7.01 \u00b1 0.03). During &gt;CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P\n                    <jats:sub>i<\/jats:sub>\n                    ] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 \u00b1 16% of baseline [PCr], 564 \u00b1 167% of baseline [P\n                    <jats:sub>i<\/jats:sub>\n                    ], and pH 6.87 \u00b1 0.10, all P &lt; 0.05 vs. &lt;CP exercise). These data support the hypothesis that the CP represents the highest constant work rate that can be sustained without a progressive depletion of muscle high-energy phosphates and a rapid accumulation of metabolites (i.e., H\n                    <jats:sup>+<\/jats:sup>\n                    concentration and [P\n                    <jats:sub>i<\/jats:sub>\n                    ]), which have been associated with the fatigue process.\n                  <\/jats:p>","DOI":"10.1152\/ajpregu.00731.2007","type":"journal-article","created":{"date-parts":[[2007,12,5]],"date-time":"2007-12-05T20:44:28Z","timestamp":1196887468000},"page":"R585-R593","source":"Crossref","is-referenced-by-count":426,"title":["Muscle metabolic responses to exercise above and below the \u201ccritical power\u201d assessed using\n                    <sup>31<\/sup>\n                    P-MRS"],"prefix":"10.1152","volume":"294","author":[{"given":"Andrew M.","family":"Jones","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Daryl P.","family":"Wilkerson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fred","family":"DiMenna","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jonathan","family":"Fulford","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"David C.","family":"Poole","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"24","reference":[{"key":"R1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1469-7793.2001.t01-1-00657.x"},{"key":"R2","doi-asserted-by":"publisher","DOI":"10.1113\/jphysiol.1996.sp021618"},{"key":"R3","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.1994.77.4.1742"},{"key":"R4","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.1994.77.5.2169"},{"key":"R5","unstructured":"Bigland-Ritchie B\n                      , Cafarelli E, V\u00f8llestad NK.\n                      Fatigue of submaximal static contractions.\n                      Acta Physiol Scand Suppl\n                      556\n                      : 137\u2013148, 1986."},{"key":"R6","doi-asserted-by":"publisher","DOI":"10.1152\/japplphysiol.00991.2004"},{"key":"R7","doi-asserted-by":"publisher","DOI":"10.1007\/BF00964125"},{"key":"R8","doi-asserted-by":"publisher","DOI":"10.1055\/s-2007-971894"},{"key":"R9","unstructured":"Conley KE\n                      , Kemper WF, Crowther GJ.\n                      Limits to sustainable muscle performance: interaction between glycolysis and oxidative phosphorylation.\n                      J Exp Biol\n                      204\n                      : 3189\u20133194, 2001."},{"key":"R10","doi-asserted-by":"publisher","DOI":"10.1113\/jphysiol.1988.sp016909"},{"key":"R11","doi-asserted-by":"publisher","DOI":"10.1152\/physrev.1994.74.1.49"},{"key":"R12","doi-asserted-by":"publisher","DOI":"10.1016\/0005-2795(70)90290-4"},{"key":"R13","doi-asserted-by":"publisher","DOI":"10.1249\/01.MSS.0000079047.84364.70"},{"key":"R14","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.1999.87.2.853"},{"key":"R15","doi-asserted-by":"publisher","DOI":"10.1152\/ajpregu.1986.251.4.R775"},{"key":"R16","doi-asserted-by":"publisher","DOI":"10.1152\/ajpregu.1983.244.4.R530"},{"key":"R17","doi-asserted-by":"publisher","DOI":"10.1249\/00005768-199510000-00012"},{"key":"R18","doi-asserted-by":"publisher","DOI":"10.1113\/jphysiol.2004.062042"},{"key":"R19","doi-asserted-by":"publisher","DOI":"10.1055\/s-2004-830335"},{"key":"R20","unstructured":"Hill AV\n                      .\n                      Muscular Movement in Man.\n                      New York: McGraw-Hill, 1927."},{"key":"R21","doi-asserted-by":"publisher","DOI":"10.2165\/00007256-199316040-00003"},{"key":"R22","doi-asserted-by":"publisher","DOI":"10.1249\/01.MSS.0000099111.78949.0E"},{"key":"R23","doi-asserted-by":"publisher","DOI":"10.1055\/s-2004-815838"},{"key":"R24","doi-asserted-by":"publisher","DOI":"10.1055\/s-2008-1025875"},{"key":"R25","doi-asserted-by":"crossref","unstructured":"Jenkins DG\n                      , Quigley BM.\n                      Endurance training enhances critical power.\n                      Med Sci Sports Exerc\n                      24\n                      : 1283\u20131289, 1992.","DOI":"10.1249\/00005768-199211000-00014"},{"key":"R26","doi-asserted-by":"crossref","unstructured":"Jenkins DG\n                      , Quigley BM.\n                      The influence of high-intensity exercise training on the Wlim-Tlim relationship.\n                      Med Sci Sports Exerc\n                      25\n                      : 275\u2013282, 1993.","DOI":"10.1249\/00005768-199302000-00019"},{"key":"R27","unstructured":"Jones AM\n                      , Doust JH.\n                      Limitations to submaximal exercise performance. 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